Tin forms two sets of salts—the protosalts and the persalts, of which the protochloride and bichlorde of tin may be taken as good examples. The protosalts (stannous salts) yield a very characteristic reaction with sulphureted hydrogen, a chocolate-colored pre cipitate of hydrated protosulphide of tin being thrown down; moreover, with a dilute solution of chloride of gold, they give either a beautiful purple precipitate, known as the purple of Cassius, or a brown precipitate of reduced gold, according to the quantity of the test that is used. The persalts (stannic salts) yield a dirty yellow precipitate of hydrated bisulphide of tin; while all the compounds of tin, when exposed on charcoal to the reducing flame of the blowpipe, give a white malleable globule of the metal.
Reduction and Nanufacture.—Tin must have been one of the metals earliest known, as it enters into the composition of bronze (q.v.), of which the most ancient metallic weapons and tools were made: Tin and oysters were the products for which Great Britain was earliest famous. This general statement of facts is particularly noticeable. Tin is still largely obtained in Cornwall; and from that locality the Phenician navigators took it to Tyre and Sidon. To this day England is by far the greatest tin-producing country, having raised in 1876 about 18,690 tons of dressed ore, or 8,500 tons of the metal. Bohemia and Saxony have some tin mines, and so also have Spain and Portugal. Tin has long been obtained from Malacca, in the Malayan peninsula, and from some of the neighboring islands. Australia, among her other mineral riches, produces tin, and the import from that country in 1876 amounted to 8,392 tons.
There is but one ore of tin 'of any importance—viz., the binoxide, or stannic oxide which in its pure state consists of tin 78, and oxygen 22. It is called tinstone or cassiterite. Tin ore has nothing remarkable in its appearance; it is of various colors— as gray, various shades of yellow, and red, and black. Its specific gravity—a notable feature—is 6.9; and it strikes fire with steel. In Cornwall the tin ore occurs in mineral veins running through granite and slate rocks, or disseminated in crystals through their mass. The tinstone obtained from the veins or lodes is called mine-tin; and that pro curred by washing alluvial deposits is called stream-tin—the latter is the result of the disintegration of granite and other rocks which contained veins of tin. Washed cornish
tin ore, usually called " black tin," produces on an average about 67 per cent of metallic or " white" tin. Tin pyrites, or sulphide of tin, is found in some of the Cornish mines, but it is of little importance commercially. It may also be stated that ores containing copper are sometimes found with so large a proportion of tin that it is difficult to say whether they should be regarded as tin or copper ores.
The dressing of tin ore obtained from the mine is a difficult and delicate operation. It is so much dispersed through the gangue, that it requires to be stamped to a very fine powder by apparatus described under METALLURGY, before the metallic particles can be effectually separated. So small, comparatively, is the valuable portion of the ore, that at Huel Kitty mine, St. Agnes, not more than 84 lbs. of oxide of tin is obtained from a ton of the material brought to the surface; and in some mines the proportion of oxide to the rest of the material is not so much as 10 lbs. to the ton.
The stamped ore is copiously supplied with water passed through a grating adjoining The stamps, and conveyed into a channel where there are two pits. The purer and heavier portion falls into the first, and is called the crop; the remainder, called the leav ings, passes through the first, and is retained in the second pit. Repeated washings are now necessary to separate as thoroughly as possible the impurities from the ore, and for this purpose a machine called a buddle is largely employed. Various kinds of apparatus are, however, used, but they are similar in principle to the jigging sieve and sleeping table described under METALLURGY. We may notice here that a new form of buddle, known as " Borlase's buddle," has been recently introduced for dressing tin ores, by which a saving of about 30 per cent is said to be effected. Fig. 1 shows this machine. The ore and earthy matters, in the state of a thick mud, are conveyed by square pipes or channels to the circumference a, a, around which, by the aid of water, the metallic portion separates, while the lighter stony impurities flow toward the center, and are carried away. There are brushes at b, b, for agitating the ore during the operation. In the older form of huddle, this action is reversed, and the machine, instead of being depressed, is raised in the center.